1. Field of the Invention
The invention relates to internal combustion engines and, more particularly, relates to a method and apparatus for reducing hydrocarbon and carbon monoxide emissions from an engine by effecting a secondary reaction between residual reactable combustion product components and supplemental air following a primary combustion event.
2. Discussion of the Related Art
Much effort has been expended in recent years to lower engine emissions to reduce urban smog. Urban smog, a severe global environmental problem, is formed by the sunlight-induced photochemical reaction of hydrocarbons (HC) with oxides of nitrogen (NOx). Because HC and NOx are both emitted by internal combustion engines, smog reduction efforts have focused on reducing these emissions. Carbon monoxide (CO), another undesired byproduct of combustion, is also an emission of concern to many researchers and engine designers. A discussion of available techniques for reducing these emissions and the problems with those techniques requires an understanding of how they are formed.
Referring to FIG. 1, the relationship between exhaust product concentrations versus equivalence ratio (ER) is graphically illustrated for a spark-ignited combustion of a homogeneous mixture of fuel and air. ER is the ratio of the stoichiometric air-fuel ratio of the air/fuel charge, divided by the actual air/fuel ratio of the charge. For example, a fuel-lean mixture having an air-fuel ratio of 29.4 and a stoichiometric air-fuel ratio 14.7 has an equivalence ratio of 0.5. A stoichiometric air-fuel ratio has an equivalence ratio of 1.0; a fuel-lean mixture has an ER value of less than 1, and a fuel-rich mixture has an ER value of greater than 1.
Curve 30 plots NOx vs. ER. NOx are formed when available oxygen and nitrogen react with one another at elevated temperatures. Generally speaking, NOx concentrations increase as the ER rises above about 0.6. However, the curve 30 also illustrates that, as the ER continues to increase beyond 1.0, the NOx concentrations fall sharply, even though the combustion temperature does not drop as sharply. This effect is due principally to the consumption of available oxygen through the reaction of the fuel and air as represented by the downwardly sloping curve 32. That is, at ERs significantly above 1.0, more fuel is available in the combustion chamber for reaction with a given volume of atmospheric oxygen. Because hydrocarbons react with oxygen more readily than nitrogen reacts with oxygen, a greater percentage of the available oxygen is consumed through combustion, leaving relatively little remaining oxygen in the combustion chamber to react with nitrogen. As a result, NOx emissions are sharply reduced as ERs rise above a stoichiometric air/fuel ratio.
Curves 34, 36, and 38 also show that CO, H2, and HC, increase steadily and non-linearly with ER due to the fact that insufficient air is present in the combustion chamber at high ERs to assure complete reaction of fuel with air during the combustion event. As a result, after combustion ceases at high ERs, the resultant combustion products have a relative high percentages of unburnt and partially burnt fuel products. Because these products are capable of oxidation under the appropriate conditions, they will hereafter be referred to as xe2x80x9cresidual reactable combustion product components.xe2x80x9d Residual reactable combustion product components form a large percentage of the undesired HC and CO emissions.
Hence, it can be seen that HC and CO emissions are proportionaly related at ERs above the stoichiometric air fuel ratio. Traditional emission reduction techniques attempted to employ fuel injection and air supply techniques to control the ER to be relatively close to 1.0 and to employ engine after-treatment in the form of a three-way converter to further reduce HC, CO, and NOx emissions. When operated very close to the stoichiometric air-fuel ratio (ER=1), the three-way catalyst has the unique ability to reduce and oxidize HC, CO, and NOx with impressive efficiency, hence reducing HC, CO, and NOx emissions to a level that the engine can reasonably be considered to be xe2x80x9ccleanxe2x80x9d or xe2x80x9cnon-polluting.xe2x80x9d The typical clean engine emits pollutant concentrations that are measured in the range of parts-per-million. Most modern automotive engines and derivatives of them can be considered to be non-polluting by this standard.
In contrast, many non-automotive engines, particularly relatively small utility engines and derivatives of them, are usually considered xe2x80x9cdirtyxe2x80x9d or xe2x80x9cpollutingxe2x80x9d because they do not incorporate active measures to reduce HC, CO and NOx emissions to the levels enjoyed by clean engines. Typical uses for these engines include, but are not limited to: lawn mowers; line trimmers, chain saws, generator sets, welding machines; cement mixers, chipper/shredder machines, mini-bikes, motorcycles, jet skis, outboard engines, and low-cost automotive engines for emerging nations. These engines are xe2x80x9crich-burnxe2x80x9d engines, typically operating at an ER value of about 1.2 or even higher. Hence, 20% of the fuel admitted to the engine passes through the engine without being combusted. The engines are factory-calibrated to run rich because they perform well at this condition and also run cooler with reduced propensity for destructive combustion knock. This, in turn, reduces a manufacturer""s warranty exposure. These engines typically produce low NOx emission levels because they operate at such a high ER.
HC and CO emissions of levels produced by utility engines and other rich-bum engines are not readily oxidized using a catalytic converter. That is, catalysts typically employed by non-polluting engines would be overwhelmed by the quantity of residual reactable combustion components emitted by a typical rich-bum utility engine. That engine is passing 20% excess fuel to the catalyst, not the trace amounts characteristic of a modern automotive engine. The reaction of 20 percent of the engine""s fuel flow within a catalytic converter generates a sizeable exothermic reaction, raising the exhaust gas temperature sharply. This high temperature can destroy the typical catalytic converter in short order.
An attempt to xe2x80x9clean outxe2x80x9d the polluting utility engine to near stoichiometric air-fuel ratio in order to reduce HC and CO emissions would also be fraught with difficulty. As briefly discussed above, this type of engine experiences compromised performance when operated at the stoichiometric air-fuel ratio. Power density, final engine weight, and cost also suffer when traditional clean technologies are employed. Design improvements to offset some of these problems would require increased compression ratio, high quality valve, valve seat, and valve guide materials, improved heat rejection schemes (likely liquid-cooling), and/or electronic ignition systems that incorporate combustion knock sensing. All of these design changes are relatively expensive to design and to implement. They also undesirably add to the weight and/or cost of the engine and the machine powered buy it. Weed trimmers, for instance, are too light-weight and inexpensive to be economically powered by a large, heavy, expensive engine.
Finally, even if a xe2x80x9cdirtyxe2x80x9d engine were reconfigured to run well at an ER that is sufficiently near an ER of 1 to reduce HC and CO emissions sufficiently for practical implementation of an oxidation catalyst, the resulting engine would produce high NOx levels that would also have to be dealt with by a three way catalyst or otherwise.
While the combustion characteristics of stratified charge engines differ from that of a homogenous charge spark-ignited engine, the underlying fundamental principals are quite similar, as are the difficulties encountered when attempting to reduce HC, CO, and NOx emissions. Similarly, while reducing HC, CO, and NOx emissions without employing fuel injectors and/or a three way catalyst and/or other extreme or expensive measures is especially difficult yet desirable in a rich-bum, spark-ignited utility engine, it is also sufficiently difficult yet desirable in a variety of other otto cycle engines and derivatives of them.
Hence, the need has arisen to provide a cost-effective, easily implemented technique for reducing HC, CO, and NOx emissions from an internal combustion engine without having to employ fuel injectors, catalysts, or other complex and/or cost prohibitive measures.
The invention differs sharply from conventional thinking in that it does not consider a lean-bum control strategy to be a prerequisite to effective HC, CO, and NOx emission reductions, nor does it require a three way catalyst or other aggressive ancillary aftertreatment equipment to reduce those emissions. The inventor reasoned that the typical OEM-supplied utility engine, as well as some other engines, start easily, run well, and emit low levels of NOx at ERs on the order of 1.2, so he decided to retain the characteristic fuel-rich engine operation and reduce HC and CO emissions in the simplest manner possible. The solution was post-combustion high-temperature residual oxidation.
More particularly, in the case of homogenous charge spark-ignited utility engine (comprising the most likely application for the invention but certainly not the only possible application), spark-ignited combustion is allowed to occur in at least generally the usual fashion (although at least preferably as rapidly as possible for a given engine design), using the standard fuel-rich carburetor calibration to fuel the engine. Then, a subsequent reaction process is initiated by supplying additional air to the engine in quantities and at times that assure reaction of the still-hot residual HC and CO products with oxygen in the supplemental air, thereby reducing HC and CO emissions.
Still more specifically, and in accordance with a first aspect of the invention, a method of reducing engine emissions comprises admitting fuel and air into a combustion chamber of an engine cylinder and then igniting the fuel to initiate an expansion stroke of the engine and to form combustion products. Then, additional air is admitted into the combustion chamber so as to react with residual reactable components of the combustion products to effect post-combustion oxidation of the residual reactable components. The resultant clean combustion products are then exhausted from the engine.
In accordance with one preferred embodiment of the invention, the engine is a 4 stroke reciprocating engine, and the post-combustion oxidizing step comprises reacting the residual reactable combustion products with air during at least one of the expansion stroke of the engine and the exhaust stroke of the engine, preferably by injecting air into an air curtain through which the combustion products pass. Air could be injected either on an xe2x80x9cearly cyclexe2x80x9d basis, i.e, timed such that the combustion products pass through the air curtain during at least part of the expansion stroke of the engine, or on a xe2x80x9clate cyclexe2x80x9d basis, i.e., timed such that the combustion products pass through the air curtain during at least part of the exhaust stroke of the engine. Preferably, the post-combustion oxidizing event is timed such that post-combustion oxidization occurs during at least part of the expansion stroke under a first engine operating condition and during the exhaust stroke during at least part of a second engine operating condition. For instance, the first and second engine operating conditions could be first and second load conditions, the second load condition being a relatively heavy load condition when compared to the first load condition.
In another practical embodiment of the invention, the engine could be a 2 stroke reciprocating engine, and the post-combustion oxidization event could take place during an expansion exhaust stroke of the engine.
In another, less practical but conceptually simpler embodiment of the invention, the engine could be a 6 stroke engine, and the post-combustion oxidization event could take place during at least one of a recompression stroke and a re-expansion stroke occurring sequentially after the expansion stroke of the engine and before an exhaust stroke of the engine.
An internal combustion engine configured to effect a post combustion reaction event as described above is also provided.
These and other advantages and features of the invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood, however, that the detailed description and accompanying drawings, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.